Displaying keyframed opacity / transparency in Material mode in viewport?
I am fairly certain this is a limitation of the current system. Simply put the changes in material settings don't trigger updates in the viewport. Manually altering values in the node editor or the material properties causes the UI to deliberately send an update event to the viewport, keyframed changes during playback don't. The keyframed changes do pass through to the render engine though.

I wouldn't expect this to change until the new dependency graph work is finished.